Current production motor vehicles, such as the modern-day automobile, are originally equipped with compartment cover assemblies that are movably mounted to the vehicle body to provide access to the vehicle's various compartments. A traditional trunk compartment, for example, is a large storage bin located at the rear of the vehicle and covered by a trunk lid that is hinged underneath the passenger compartment's rear deck. Driver-side and passenger-side vehicle doors, on the other hand, can be opened and closed to allow user access for entering and exiting the passenger compartment. In contrast, the hood (or “bonnet” in some countries) extends over and covers the vehicle's engine compartment to prevent theft and damage of the engine components. On passenger cars, the engine hood is typically hinged to a forward cross-member underneath the dashboard panel or to lateral engine compartment rails of the body in white (BIW), and secured to a front bulkhead cross-member via a releasable latching mechanism.
Engine hoods are typically constructed using stamped metal panels, which combine substantial overall strength and stiffness with a smooth, paintable exterior surface. Hood stiffness is often satisfied via the combination of a relatively high strength stamped metal (outer) fascia panel—often referred to as an “A-surface”—coupled with a preformed inner reinforcement panel—often referred to as a “B-surface”—supported by a series of engine-side hat-section reinforcements. The hat-section reinforcements are typically positioned between the inner and outer panels of the hood, and include a pair of upper flanges oriented toward the A-surface as well as a single lower flange oriented toward the B-surface, with the upper and lower flanges interconnected by a web portion. This conventional hood construction increases the bending stiffness of the engine hood by placing relatively stiff material, usually stamped aluminum or steel, away from the neutral axis of bending of the hood.
In certain vehicle impact scenarios, an object may exert a downward force on the hood; engine hoods are typically designed to deform when a downward force is exerted thereto. However, the deformability of the hood and, correspondingly, the hood's ability to absorb energy may be impeded by the proximity of the hood to rigidly mounted components housed in the vehicle's engine compartment. By way of example, the hood's ability to absorb energy through deformation can be significantly impeded where the hood and engine block are in close proximity. However, minimal clearance between the hood and the engine compartment components may provide significant benefits, such as improved driver visibility, increased aerodynamics, and desired aesthetic appeal. In contrast, additional clearance between the vehicle hood and engine compartment can increase the hood's ability to absorb energy when acted upon with a downward force.
To maintain a low hood line during vehicle operation yet achieve greater clearance between the hood and the engine bay contents during an impact event, many vehicles are provided with an active hood system (referred to colloquially as a “pop-up hood”) that is operable to change the orientation and/or position of the hood with respect to the engine prior to deformation of the hood. As an active vehicle system, the engine hood, in response to an impact event, is automatically tilted, raised or otherwise displaced away from the engine by means of spring-loaded or pyrotechnic actuators. These actuators are activated by an impact-detecting sensor arrangement that is packaged at the front end of the vehicle, normally between the outer bumper fascia and the inner bumper crossbeam. The impact-detecting sensor arrangement may include a pair of pressure sensors connected to opposing ends of an air tube that is packaged behind a crushable foam block. The pressure sensors output a trigger signal when the air tube is compressed by a crushing force delivered to the foam block through the bumper fascia at the time of impact. Sensor signals can provide various types of impact information, such as impact type, impact angle, and severity of impact, to an onboard electronic control unit (ECU). Using this information, the ECU employs stored impact algorithms to determine if the impact event meets threshold criteria for deployment; if so, the ECU responsively triggers firing circuits to activate the actuators and thereby deploy the engine hood.